Hydrologic versus Hydraulic Rainfall–Runoff Modeling of New Jersey Watersheds

Rainfall–runoff modeling is commonly employed by hydrologic engineers for the purpose of determining a runoff hydrograph. Historically, hydrologic methods utilizing unit hydrograph theory and basin averaged parameters have been used to obtain results. Of the available unit hydrograph methods, the Natural Resources Conservation Services (NRCS) procedure has gained widespread popularity and is commonly used by the professional community. Recent advancements in both computing power and accessibility to high-resolution digital elevation models (DEMs) have made 2D hydraulic rainfall–runoff modeling feasible. Version 6 of the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) allows users to input a precipitation boundary condition used in combination with both curve number and Manning’s n layers for the purpose of modeling the rainfall–runoff relationship over a 2D terrain using hydrodynamic equations. The goal of this study was to simulate runoff hydrographs using both a hydrologic method (NRCS, via HEC-HMS) and a hydraulic method (via HEC-RAS) and then to compare results to measured values recorded at a USGS gauging station. A total of 29 watersheds were analyzed. Eleven of the watersheds had more than one historical storm, leading to a total of 45 rain events that were investigated. Analysis showed that neither technique produced consistently accurate results compared to historical data. The hydraulic (HEC-RAS) simulations produced average peak flow and runoff depth errors of 103% and 67% respectively, with 23 and 21 of 45 simulations overestimating the peak flow and total runoff depth, respectively. The hydrologic (HEC-HMS) simulations had average peak flow and runoff depth errors of 105% and 81% respectively, with 28 and 31 out of 45 simulations overestimating the peak flow and total runoff depth. In general, peak flow results for both models tended to improve for simulations involving larger watershed areas, larger flow return periods, and larger rainfall events. Total runoff depth results tended to improve only for larger rainfall events.

This paper shows that rainfall–runoff modeling using a hydraulics-based approach yields better results with respect to both peak flow and direct runoff estimations. This is significant because if adopted by the professional community, it would lead to a fundamental shift in the approach that the practicing engineering community takes while performing this sort of analysis (peak flow prediction used for hydraulic structure design, volume estimation used for the reservoir, and stormwater management design). As higher resolution terrain information as well as various watershed features become more readily available, the results of the hydraulic modeling should theoretically improve. Ultimately, standard hydrologic methods involving unit hydrograph theory may become obsolete.